Added test file for CBlockIndex, including a test for get block header.

Summary:
CBlockIndex has no tests of it's own. This partially completes T355 (part 1 of many).
This test specifically is just a sanity check to make sure GetBlockHeader returns what it's given.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1533